1. Field of the Invention
The present invention is directed to blends of waste and useful materials in the steelmaking process, and a process for preparing such recycle blends. More specifically, the present invention is directed to recycle blends of heretofore waste materials and useful materials that can enhance the BOF steelmaking process, and the process for making the recycle blends to enhance steelmaking operations.
It is well known in the art of steelmaking that iron bearing dust and sludges generated by steelmaking furnaces and related steel processing facilities contain valuable materials useful in the steelmaking operations. Such waste materials contain iron oxides in an amount up to about 50% by weight, and it is very desirable to recover the iron for use as a steelmaking charge material. In the case of wet sludges, and especially in the case of wet BOF scrubber sludge, high moisture content has made the wet sludge very difficult to handle in a recycling stream. These sludges also contain high proportions of iron oxides that also are desirably recovered through recycling to the steelmaking furnace.
Filter cake produced from wet scrubber typically has a moisture content of about 30% by weight, or higher. The sludges have a relatively high viscosity, causing them to have poor handling characteristics. The sludges stick to conveyors and machinery when attempts are made to convey them as reverts in a recycling stream. They often form sticky agglomerations that clog and shut down equipment and machinery. For example, under test conditions, it has been found that wet sludge having a moisture content of  greater than 10% has a flow rate of less than 10 pounds of sludge per minute. Such a low flow rate makes these wet materials very unsuitable for recycling as a steelmaking revert.
Steelmaking dusts are relatively dry, and powdery. In instances where waste steelmaking dust is recycled, the dry powdery conditions of the material may cause environmental dusting problems. To control dusting, water is normally applied to the dry material, thus increasing the moisture content and potentially creating the handling problems attributable to sludges. Attempts to produce a steelmaking revert by combining wet sludges with dry steelmaking dusts have generally been unsuccessful because such reverts introduce high levels of undesirable elements and compounds into the steelmaking process. For example, if sludge from a hot dip coating line is introduced into a recycle stream, zinc levels in the final revert material can be raised to where the blended revert is unsuitable for use in a steelmaking operation. Combining steel plant wastes must be carefully monitored for chemistry to avoid introducing deleterious elements into the steelmaking process.
2. Brief Description of the Related Art
Various apparatus and methods have been developed in the past to reduce the moisture content and/or recover iron from wet sludges. For example, U.S. Reissue Pat. No. 30,060 teaches a process that instantaneously vaporizes the water in sludges by spraying the sludge into a hot (1200xc2x0 F.) gas stream. U.S. Pat. Nos. 4,091,545 and 4,133,756 teach using a hot gas to reduce the moisture content of wet sludge. U.S. Pat. Nos. 5,114,474, 4,725,307, 4,711,662 and 2,710,769, teach mixing wet sludges and dust with molten slag to produce reverts. The mixture is crushed for recycling after the slag cools and solidifies.
An article in xe2x80x9c33 METALPRODUCING,xe2x80x9d March 1997, discloses a process that forms BOF waste sludge into briquettes. The apparatus used in the process includes a rotary kiln or dryer to remove water from the sludge, a roll-press, screw conveyors, and pug mills. A recycling plant of that sort requires a large capital investment. Moreover, the rotary kiln consumes energy to generate heat for drying the sludge, thus increasing operating costs. The article also discloses using heated molasses as a binder to form the briquettes. The heated molasses also adds cost to the recycling process.
U.S. Pat. No. 5,871,561 discloses a process wherein steelmaking dusts containing iron oxides generated from electric furnaces are formed into briquettes with reducing agents. The briquettes are fired in furnaces to obtain metal that can be recycled as steelmaking material. The briquettes are made either by a press machine or by the addition of binder material. Both means of making the briquettes increase the cost of steelmaking.
U.S. Pat. No. 5,785,737, to Lynn et al., assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, solved the foregoing problems. The process of the ""737 Lynn patent provides a steelmaking revert with improved flow rate properties in a recycling stream. Wet sludge and hot slag are combined into a hot slag/sludge mixture, after which the hot slag/sludge mixture is allowed to rest for a period sufficient to allow the hot slag to cause water vaporization and reduce the moisture content of the slag/sludge mixture to  less than 10% by weight. The dehydrated slag/sludge mixture may be recycled as a steelmaking revert. Although the foregoing problems associated with steelmaking were successfully addressed by the ""737 patent, there is still a need to further enhance steelmaking operations.
Steel manufacturing operations are costly, and the high manufacturing costs extend to the procurement of raw materials. The process of mining and beneficiating ores and raw materials is a very labor intensive and expensive part of the steelmaking process. Further, disposing of the waste from the hot steelmaking operations as well as from the downstream and finishing operations, can create environmental problems. For example, fine powdery kish produced during steel making may disperse into the air unless appropriate steps are taken. Although the carbon content of the kish suggests that it may be recycled into the steelmaking operations, the fine consistency of the kish makes handling very difficult. The low mass of the kish prohibits it from penetrating the frothy, slag covered surface of the steelmaking furnace, causing it instead to blow out of the furnace. Such a problem is not specific to kish, but to any fine material which has a low mass.
In addition to kish, other waste materials contain raw materials that are used in steelmaking, but like kish are difficult to recycle because of their physical form or low mass. Ferro-manganese is a waste material that has proven difficult to recycle. Ferro-manganese (about 76% Mn, about 15% Fe) is used during steelmaking as a deoxidizer. Addition rates of ferro-manganese to steelmaking furnaces amount to about one ton per heat. Recovery of ferro-manganese from slag rich in ferro-manganese results in low grade fines (about 50%). Ferro-manganese slag has not been suitable for recycling because of the handling required. Other waste materials that are difficult to recycle include glass from municipal recycling facilities, and the like. Such waste materials typically are landfilled, thus creating additional environmental problems.
Landfills are costly to municipalities and steel plant operators, and present environmental hazards. If steelmaking and other waste material could be recycled, the cost to municipalities could be reduced. In addition, environmental hazards would be prevented. Additionally, recycling such waste materials would benefit steelmakers by reducing the high costs associated with procuring raw steelmaking materials. Accordingly, there is still a need to enhance steelmaking operations, and a need to recycle waste material.
The present invention is directed to a recycle steelmaking revert composed of a carrier containing a slag/sludge mixture blended with other materials and waste materials useful in the steelmaking process. The recycle materials include, but are not limited to, kish, glass, deoxidizing agents such as ferro-manganese, lime fines, and the like. The specific recycle materials are blended into the slag/sludge mixture individually, or the specific recycle materials are blended into the slag/sludge mixture in different combinations. The steelmaking revert comprising the recycle material blended into the slag/sludge carrier preferably has a moisture content of  less than 10% by weight and  greater than 4% by weight in order to preclude undesirable dusting.
The present invention is also directed to a method of preparing the recycle blends. The process includes the steps of blending one or more of the recycle materials into wet sludge and hot slag, followed by combining the wet sludge and the hot slag with the blended recycle materials into a recycle blend hot slag/sludge mixture. The recyclable blend hot slag/sludge mixture is allowed to rest for a period sufficient to allow the hot slag to vaporize and reduce the moisture content of the mixture to  less than 10% by weight. The recycle blend slag/sludge mixture may then be recycled as a steelmaking revert or combinations of recycle blends may be recycled as a combination revert as steelmaking operations may require.
Advantageously, the recycle blend slag/sludge mixture and process for making it enhance steelmaking operations. Difficult to handle recycle materials are readily blended in slag and sludge to form a recyclable steelmaking revert that can be immediately used, or can be conveniently stored for later use. Recycling of the materials replaces steelmaking raw materials, and the cost of obtaining such raw materials. Additionally, the present invention eliminates disposal problems for waste materials both for the steel industry and for municipalities. Additionally, the prior costs for landfilling waste materials are thus eliminated, permitting the steelmaking operation to be more productive and efficient. The wastes now become valuable commodities.
A primary object of the present invention is to provide a steelmaking revert formed from difficult to handle waste and other useful materials.
Another object of the present invention is to provide a steelmaking revert that eliminates costly disposal problems.
An additional object of the present invention is to provide a steelmaking revert that can be readily stored.
Still yet another object of the present invention is to provide a steelmaking revert that contains specific recycle materials that may be selectively added to the steelmaking furnace alone or in combination as steelmaking operations may require.
These and other objects and advantages of the present invention are readily apparent in view of the following description and drawings.